The present invention relates to a macro lens system for a camera, particularly, for an auto-focus camera.
As an auto-focus (automatic focus) system, a TTL (Through The Lens) auto-focus system, which utilizes light passed through a photographing lens of a camera for an auto-focus operation, has been known. The TTL auto-focus system requires that the camera is provided with a lens having a small F-number. Generally, if an effective F-number at a full-open aperture is larger than "7", light amount received by a sensor of the auto-focus system is too small for reliable operation.
As one of photographing lenses, a macro lens is known. In the macro lens, when the focusible range thereof is broadened, the effective F-number thereof varies within a relatively wide range. That is, the effective F-number is maximum when the lens is set to focus on an object at the minimum object distance, and the effective F-number becomes minimum when the lens is set to focus on an object at the infinity. The maximum F-number of the macro lens may exceed the upper limit required by the auto-focus system if a reasonable lens size is to be maintained.
In an example of a conventional macro lens for a middle or large format camera, the minimum F-number is "4" at infinity and the maximum F-number is "8" at the minimum object distance. This maximum F-number is too large for the auto-focus system to operate.
If the conventional macro lens is designed so that the maximum F-number is approximately equal to "6", the minimum F-number becomes approximately "3". Although such a macro lens allows operation of the auto-focus system even at the minimum object distance, the size of the lens (i.e., diameter and length) will become too large to use practically. It is because, at a given focal length, the smaller the minimum F-number is, the larger the lens diameter becomes to keep the light amount, and the larger the number of lenses are required to reduce aberrations.